The Dealey Plaza is a landmark in the city of Dallas because that's where JFK died in 1963. The square, a green space with grass and trees, is traversed by three streets: Commerce Street, Main Street and Elm Street. It's in between Houston Street and Triple Bypass Highway. In the same square there's also '"The sixth floor museum", from which the former U.S. was shot. As a curiosity, note that on the road they have marked the point where JFK was shot, although this is best seen in the accompanying video.
The unique Pioneer Plaza, situated in the center of Dallas, is where you can find the largest bronze statue in the world. It represents (very realistically) American cowboys driving cattle. The rest of the Plaza is without interest: ten minutes, some pictures and you are ready to go. Incidentally, Pioneer Square has a small parking.
Thanks-Giving Square is a great place to slow down, and appreciate the good things in your life. It's religious ambiguity is a deliberate attempt by the founders to avoid any one affiliation. The square is meant to be a place for anyone, regardless of belief, to come and take a moment to give thanks for the blessings of our lives. The chapel is small, but absolutely breathtaking when you step inside. You may leave a note in the bowl on the alter giving thanks, or take a moment to pray or meditate. The Thanks-Giving Foundation encourages you to lay on the floor, and look upwards. You will see a spiral of 73 stained-glass panels that make up the Glory Window. As the brochure explains, it is the largest display of horizontal stained glass in the world. The Foundation's web-site further describes that in the year 2000, the United Nations declared it the year of International Thanksgiving. The Glory Window appeared on the UN Stamp.
After spending a moment in the chapel, take some time to wander the square. Take a moment to appreciate the tranquility of the gardens. Make your way to the Bell Tower, and step inside the Ring of Thanks. If you turn toward the ring, you can hear your expressions of thanks echo!
The square is easily accessible by DART, or by driving. This is not an all day event, but definitely worth spending a good hour. Check the website for special events. There are services that take place occasionally, and all are invited to join. Check the website for special events and hours.